A pump assembly of the type above-described is disclosed in U.S. Pat. No. 4,029,442 to Schlosser and U.S. Pat. No. 4,035,109 to Drath and Schlosser. Both patents include a disclosure of a single-acting pump having a piston that is smaller than and spaced from the walls of the pump cylinder chambers and that moves into and out of a piston sealing structure during the reciprocal movement of the pump piston in its forward or pumping stroke and its return or suction stroke. The sealing structure helps maintain the piston spaced from the walls of the cylinder chambers and provides the only surface in the pump with which the piston engages so there is no metal to metal contact, and therefore, reduced friction and wear. The two patents disclose respectively different cylinder mounted sealing means for bridging the space or gap between the piston and the walls of the associated cylinder chambers on the pressure or pumping strokes of the piston. In the patented device, the pump piston is withdrawn completely from its seal on its rearward or suction strokes and is forced back through the seal on its forward or pumping strokes. This mode of operation causes a suction force to be generated in the pumping chamber on the suction stroke of the piston, thereby to induce flow of viscous material into an intake chamber rearwardly of the pumping chamber and thence into the pumping chamber as the piston releases from its seal.
With the intake chamber located upstream, i.e., rearwardly, of the pumping chamber, the piston rod of the piston extends through the fluent material in the intake chamber to the piston actuating air motor and has to be very effectively and efficiently sealed to the air motor to prevent entry into the air motor of the fluent material being pumped, and to prevent entry of air into the intake chamber which would render the pump inoperable.
If the piston is of greater diameter than the piston rod, as is usually the case, the back and forth movement of the piston within the intake chamber causes pulsation or surging of the fluent material in the intake chamber, which consumes some of the energy imparted to the piston by the air motor, and imposes a high stress load on the seal between the air motor and the piston rod. To mitigate the problems thus generated, the pump has no inlet valve and/or has surge absorbing or compensating means in the inlet to the pump, so that the fluent material can surge or pulse back and forth within the intake chamber and the inlet to the pump without imposition of undue pressure on the seal. Design considerations thus inhibit or prevent pressurized or force feeding of fluent materials into the pump inlet.
Because of these design considerations and the viscous/abrasive nature of the fluent material, the only commercially practical seal for the piston rod is a bellows seal. Use of a bellows seal further inhibits preloading or force feeding of the fluent material into the intake chamber because any significant degree of pressure would collapse the bellows, rupture the seal, render the pump inoperable, and potentially cause extensive damage to the air motor components.
For these reasons, the prior art pump is essentially dependent upon the suction force generated by the retracting piston as it releases from its seal, and cannot tolerate a force-fed or pressurized source of fluent material at the pump inlet. This limits the number of materials that can be handled by the pump and the number of systems applications in which the pump can be successfully employed.
Despite their many shortcomings, pumps of the type described have enjoyed commercial success, in the form particularly of the "Glutton" pumps sold by Graco, Inc. and the "Funny" pumps sold by Binks Manufacturing Company, the assignee of this application.
Other patents disclosing single acting pumps having operational characteristics the same as or similar to the pump described include Medo, German Gebrauchsmuster (utility model patent) No. 1,826,851; Penn, British Patent No. 1,357,961; Stallsworth, U.S. Pat. No. 1,043,267; Santarelli, U.S. Pat. No. 2,569,903; Corneil, U.S. Pat. No. 2,786,656; and Roeser, U.S. Pat. No. 3,802,805.
Another type of single acting pump is disclosed in U.S. Pat. No. 3,174,409 to Hill. The pump disclosed in the Hill patent differs from the above-described prior art pumps in that it does not have an intake chamber rearwardly of the pumping chamber, utilizes a piston/piston rod of uniform diameter smaller than the diameter of the pumping chamber, which is in continuous engagement with and does not leave its seal, and has a pumping chamber provided with an inlet valve as well as an outlet valve. However, in Hill, the seal between the piston/piston rod and the pumping chamber also separates the pumping chamber from the air motor actuator, with the consequence that any leakage past the seal will result in ingestion of air into the pumping chamber on the piston suction stroke and pumping of fluent material into the air motor on the piston pressure stroke. Also, due to the small diameter and long stroke of the piston, and the small diameters of the fluid receiving chambers, fluent material passing through the chambers will have a flow path in the shape of a "Tee", which is not conducive to pumping viscous or shear sensitive materials. Consequently, even though Hill discloses in-line inlet and outlet valves leading to a common pumping chamber, including a piston spaced from but having a sealed relationship with the walls of the chamber, the pump of the Hill patent suffers too many disadvantages to be suitable for heavy-duty industrial use.
Other patents showing in-line inlet and outlet check valves, in different pump environments, include Browne, U.S. Pat. No. 2,625,886, Smith, U.S. Pat. No. 3,318,251, and Rawicki, U.S. Pat. No. 4,178,133.
An additional prior art disclosure of interest is the Huber U.S. Pat. No. 3,233,544. This patent discloses an air compressor having a pair of interconnected opposed pistons operating within respective cylinder chambers, wherein the pistons are spaced from the walls of the cylinder chambers, engage only their respective seals and have no metal to metal contact with the cylinder chambers. As in Hill, the pistons are maintained in continuous engagement with their seals and the pumping chambers are provided with both inlet and outlet valves. However, in Huber, as in Hill, the seal between each piston and its pumping chamber also separates the pumping chamber from the actuator chamber, and the fluid flow path is restricted and tortuous. If an attempt were made to pump a fluent material with the Huber design, it would suffer the same shortcomings and would be just as unsuitable for industrial use as the pump of Hill.